1. Field of the Invention
The present invention relates to dispersion stabilization of ink compositions with a dihydroxybenzophenone or a derivative thereof and a method for preparing stabilized ink compositions.
2. Discussion of the Background
A class of water-dispersible polymers described in U.S. Pat. No. 3,734,874, U.S. Pat. No. 3,779,993 and U.S. Pat. No. 4,233,196 (which are hereby incorporated by reference in their entirety) have found a number of commercial uses, but have mainly been employed heretofore as textile sizing materials.
These polymers are, for the most part, water dispersible because they form electrostatically stabilized colloids when mixed with water. The colloid particle size varies with the polymer composition but has been shown by light diffraction studies and transmission electron microscopy (on fresh films) to be mostly 200-800 .ANG. in diameter. When maintained in the temperature range 0.1.degree. C.-99.9.degree. C., the aqueous colloid dispersions demonstrate a minimum precipitation of solid material with time. This is because the relationship between the particle densities and viscosities (very similar to those of water when concentrations are less than 30 weight percent) are such that thermal energy expressed as Brownian motion is sufficient to keep the particles suspended in water.
The fact that these polymers are not truly "dissolved" in water is readily shown by the facts that: 1) the viscosity of the dispersion is quite low and Newtonian, despite high polymer molecular weights, until relatively high concentrations (greater than 30 weight percent in water) are reached, and 2) addition of certain salts can compromise the electrostatic stability and completely precipitate the polymer from water. This second property is useful and is the basis for a process for treating ink wastes, as described in U.S. Pat. No. 4,738,785 (hereby incorporated by reference in its entirety).
It has been found that these water dispersible polymers act as film forming dispersing agents that can be used to formulate inks (see, U.S. Pat. No. 4,148,779, hereby incorporated by reference in its entirety) because they can be used to disperse significant amounts of water insoluble colorants, both dyes and pigments.
It has also been found that other water insoluble materials can be sequestered into aqueous dispersions of these polymers (see U.S. Pat. No. 4,335,220 hereby incorporated by reference in its entirety). These materials must be in the liquid state to achieve sequestering, as taught by that disclosure.
For many common printing applications, especially commercial printing using low viscosity inks such as rotogravure and flexography, the improved ink technology described in U.S. Pat. No. 4,148,779 and U.S. Pat. No. 4,335,220 gives excellent formulations with significant advantages in clean up, pollution control and worker exposure, as well as bright high-value printed products. It is possible to formulate four color inks in the substractive primary colors (yellow, magenta, and cyan), as well as black, and to print using color-separated originals and, with existing machinery run by those skilled in the art, to achieve excellent four color printed products. Also using that technology, it is possible to create inks with colorant blends and/or to blend existing inks to give specific colored inks--for example, to achieve an ink color associated with a particular product's trade dress and to have those inks give the same desirable results already described.
However, the problem with all low viscosity inks is that the colorant materials will rapidly precipitate from the ink. This is not a new problem. Machinery designed to print with low viscosity inks include stirred ink storage tanks and elaborate ink pumping systems to circulate ink and to prevent settling of the colorants. Even these measures are not always successful and the resulting colorant "sludge" is another disposal problem from most such commercial printing operations. The energy expended and extra mechanical complexity would not be needed if inks were available that were essentially free from colorant precipitation. Moreover, there are other markets (inks for low viscosity writing instruments, inks for computer run plotters, inks for continuous and drop-on-demand ink jet machines, etc) where aqueous inks would be preferred and where precipitation of the colorant cannot be prevented mechanically and cannot be tolerated in practice.
Thus, there remains a need for aqueous ink compositions from which the water-insoluble colorant material does not rapidly precipitate.